Eu₉Cd_4–xCM_2+x–y□_ySb₉: Ca₉Mn₄Bi₉-Type Structure Stuffed with Coinage Metals (Cu, Ag, and Au) and the Challenges with Classical Valence Theory in Describing These Possible Zintl Phases

Abstract

The synthesis, crystal structure, magnetic properties, and europium Mössbauer spectroscopy of the new members of the 9-4-9 Zintl family of Eu(9)Cd(4-x)CM(2+x-y)□(y)Sb(9) (CM = coinage metal: Au, Ag, and Cu) are reported. These compounds crystallize in the Ca(9)Mn(4)Bi(9) structure type (9-4-9) with the 4g interstitial site almost half-occupied by coinage metals; these are the first members in the 9-4-9 family where the interstitial positions are occupied by a monovalent metal. All previously known compounds with this structure type include divalent interstitials where these interstitials are typically the same as the transition metals in the anionic framework. Single-crystal magnetic susceptibility data indicate paramagnetic behavior for all three compounds with antiferromagnetic ordering below 10 K (at 100 Oe) that shifts to lower temperature (<7 K) by applying a 3 T magnetic field. (151)Eu Mössbauer spectra were collected on polycrystalline powder samples of Eu(9)Cd(4-x)CM(2+x-y)□(y)Sb(9) at 50 and 6.5 K in order to evaluate the valence of Eu cations. Although the Zintl formalism states that the five crystallographically distinct Eu sites in Eu(9)Cd(4-x)CM(2+x-y)□(y)Sb9 should bear Eu(2+), the Mössbauer spectral isomer shifts are clearly indicative of both 2+ and 3+ valence of the Eu cations with the Cu- and Au-containing compounds showing higher amounts of Eu(3+). This electronic configuration leads to an excess of negative charge in these compounds that contradicts the expected valence-precise requirement of Zintl phases. The spectra obtained at 6.5 K reveal magnetic ordering for both Eu(2+) and Eu(3+). The field dependence of Eu(2+) indicates two distinct magnetic sublattices, with higher and lower fields, and of a small field for Eu(3+). The site symmetry of the five Eu sites is not distinguishable from the Mössbauer data.

Non-stoichiometric compositions arising from synergistic electronic and size effects. Synthesis, crystal chemistry and electronic properties of A14Cd1+xPn11compounds (0 ≤ x ≤ 0.3; A = Sr, Eu; Pn = As, Sb)

“…The Cd atoms occupy the interstitial sites forming distorted [CdBi 4 ] tetrahedra sharing edges and faces with the neighboring distorted [CaBi 6 ] octahedra and [CaBi 5 ] square pyramids. Finally, it is important to note that the structure of the A 9 M 4+x Pn 9 family offers more structural flexibility to accommodate new cationic species leading to stuffed variant compounds as evidenced by the recent report of Kazem et al 54 In the next sections we will attempt to gain further insights into two main questions: (1) why the interstitial chemistry in A 14 M 1+x Pn 11 appears to be limited to the Sb-compounds, while the As-analogs show very little or no occupation at the interstitial site (but show different types of structural distortions/ variances)? and (2) given the very low electron density at the interstitial position and the lack of conclusive evidence from the interatomic distances, ¶ would the formula A 14 M 1+x Pn 11 (i.e., interstitial site filled by Cd 2+ ) or A 14 MPn 11+x 12 (i.e., assign Sb 3À…”

Sr9Mg4.45(1)Bi9 and Sr9Mg4.42(1)Sb9: Mg-Containing Zintl Phases with Low Thermal Conductivity

“…This phenomenon has been well discussed in most 9−4−9 phases, and the deficiency is determined by size and electronic effects. 31,33,35 According to the previous studies on the Zn-containing ananlogue Ca 9 Zn 4+x Sb 9 , these intertitial atoms are critical in tuning the electrical transport properties, and a higher content can significantly improve the carrier mobility of the materials. 34 Thus, the thermoelectric measurements on Sr 9 Mg 4.45(1) Bi 9 will be especially interesting, and the results are discussed below.…”

“…Besides Yb 14 MnSb 11 , which has a high-temperature thermoelectric performance superior to the traditional SiGe alloys with a maximum figure of merit (ZT) over 1.0, − many other Zintl analogues have also shown potential for very promising applications. − On the basis of the above progress, the Mg-containing analogues reported in recent years are considered as potential thermoelectric materials, such as A 14 MgPn 11 (A = Yb, Sr, Eu; Pn = Sb, Bi) related to the 14–1–11 system − and Mg 3 Sb 2 -based materials. , What’s more, a huge family with the nominal A 9 TM 4+ x Pn 9 formula (A = Ca, Sr, Eu, Yb; TM = transition elements = Zn, Cd, Mn; Pn = As, Sb, Bi) was also intensively investigated during the past decades. − …”

Sr/Eu cation doping and thermoelectric properties of the compound Ca&lt;sub&gt;9&lt;/sub&gt;Zn&lt;sub&gt;4.5&lt;/sub&gt;&lt;sub&gt;&minus;&lt;/sub&gt;&lt;sub&gt;&lt;italic&gt;&delta;&lt;/italic&gt;&lt;/sub&gt;Sb&lt;sub&gt;9&lt;/sub&gt; (0&lt;&lt;italic&gt;&delta;&lt;/italic&gt;&lt;0.5)

“…Zintl相有着复杂的晶体结构和较低热导率的特点, 这使其在高温热电能源发电领域表现出了较高的潜 能 [19,20] . 在过去的数十年中, 大量Zintl相被研究和扩展 [34,35] , Eu 9 Cd 4 Sb 9 [36,37] 和Ca 9 Zn 4.5−x Cu x Sb 9 [33] 化合物材料中取得的最大ZT值可以达到0.7. 最近报道 的一篇Ca 9 Zn 4.5−δ Sb 9 研究中, 通过第二相固溶的方法改 变了化合物中化学势能进而改变了化合物成分, 增大 了过渡金属Zn的含量, 最终在875 K得到了1.1的ZT 值 [38] .…”